1. Field of the Invention
This invention relates generally to RF antenna, and more particularly to high precision shaped reflector optimization for antenna.
2. Description of the Related Art
Today, spaceborne satellite systems are used to transmit and receive electromagnetic energy for communication and other purposes. To focus the electromagnetic energy, satellite systems frequently use an antenna assembly that includes a main reflector and a feed assembly located at a focal point of the main reflector. In operation, the feed assembly illuminates the main reflector with an electromagnetic energy beam. The main reflector then reflects and focuses the electromagnetic energy beam into a radiation pattern for transmission to Earth. Similarly, the main reflector focuses impinging electromagnetic energy from a radiation pattern into a reflected beam on the feed assembly when the antenna assembly is receiving a signal.
It is advantageous to reduce the amount of wasted power in satellite antenna because of the extreme losses caused by the transmission distance. Power is wasted when unwanted areas on the Earth's surface receive a portion of the transmitted signal. Hence, advanced antenna designs are tuned to the desired coverage region so that as much power as possible is gathered from the region while little or no power is gathered from outside of the region. Accordingly, antennas generally are designed to transmit and receive signals having radiation patterns contoured to fit the shape of a desired coverage region. For example, the desired coverage region may be Europe, the continental United States, or a group of cities.
The current state of the art for producing shaped contour radiation patterns is to use a shaped main reflector. A shaped main reflector is a main reflector that has had its surface shaped to produce a desired radiation pattern. Antenna systems frequently employ a shaped reflector to collimate or focus a beam of energy into a selected shaped beam pattern with high radiation efficiency. In doing so, a feed horn is generally employed to communicate with the shaped surface contour of the reflector to radiate energy off the reflector and/or receive energy therefrom. Hence, a shaped reflector advantageously allows the use of a single feed horn to obtain the desired beam pattern. Various methodologies are currently used to generate and build a shaped reflector surface, as illustrated in FIG. 1.
FIG. 1 is a flowchart showing a prior art method 100 for generating a shaped reflector. In an initial operation 102, preprocess operations are performed. Preprocess operations can include, for example, determining desired coverage area for a particular antenna usage, determining RF signal requirements based on the desired coverage area, and other preprocess operations that will be apparent to those skilled in the art.
In operation 104, an ideal reflector surface is generated based on predefined RF requirements. With the predefined RF requirements set, a reflector surface shape is defined. This generally is accomplished using an RF optimizer, which is a computer program capable of generating reflector surfaces to meet a set of performance requirements over a prescribed coverage region. RF optimizers operate on math models of the reflector surface definition, which generally is initially defined as a parabola. It is important to note that the same coverage region can be produced from a virtually infinite number of different reflector shapes; there is no single mathematically perfect solution which will achieve the desired coverage.
With no knowledge of the existence of surface tuners or of the precision of the reflector surface currently in assembly, an RF optimizer arbitrarily manipulates the math model of the surface to produce a modified surface shape. The RF optimizer then predicts the performance of the modified surface and compares the predicted performance with the predetermined RF requirements to determine if the modification is beneficial. If the modification is beneficial, the modification is incorporated. This process is repeated through much iteration until the predicted performance is optimized for the predetermined RF requirements.
In operation 106, a shaped reflector is designed and built to closely match the ideal reflector surface generated in operation 104. Typically, space qualified techniques are used to manufacture the shaped reflector to achieve a shape very close to the ideal reflector surface. Generally, the reflector comprises a carbon fiber or KEVLAR (polyamide) composite RF reflective surface mounted to a backing structure.
Once the shaped reflector is built, the shaped reflector surface is measured in operation 108. One technique used for measuring the surface of a shaped reflector is photogrammetry. Photogrammetry is a 3D measurement technique wherein reflective targets are placed on the shaped reflector surface and a series of shots are taken using a camera that includes a stroboscope. Using control points and triangulation points, software bundles the measured data and the geometry of the shaped reflector surface is reconstructed with very high precision.
In operation 110, the measured shaped reflector surface is compared to the ideal reflector surface. Unfortunately, as the reflector is being built, the shaped reflector surface drifts from the ideal shaped reflector surface due to several factors. For example, the build-up of internal stresses that result with the materials used in the manufacturing process is one such factor. Hence, due to a number of factors, the measured shaped reflector surface will be different than the ideal reflector surface.
To compensate for these differences, in operation 112, the shaped reflector surface is tuned to better approximate the ideal reflector surface. When manufacturing the shaped reflector, tuners are located at discrete points along the backside of the shaped reflector surface. The tuners provide adjustability by displacing the surface normal to the surface's defined surface plane. Hence, the tuners are used to adjust the shaped reflector surface to closer resemble the originally prescribed ideal reflector surface generated in operation 104. Post process operations are then performed in operation 114 that normally include re-adjusting the feed/reflector geometry relationship analytically to accommodate the inaccuracies of the final as-build reflector. Post process operations can also include, for example, redesigning the reflector surface if the surface cannot be adjusted to perform properly, and other post process operations that will be apparent to those skilled in the art.
Very high performance antenna systems rely on very tight surface tolerances to meet their requirements. As the performance requirements increase, higher and higher reflector surface precision is needed to meet the increased performance requirements. Unfortunately, in modern spacecraft antenna systems the accuracy that can be maintained with the prior art methodology outlined in FIG. 1 limits the antenna's ability to meet these stringent performance requirements. In view of the foregoing, there is a need for techniques capable of generating shaped reflectors with increased precision.